The present invention relates to a film for high heat-sensitive stencil paper and, particularly, to a film for high heat-sensitive stencil paper which has an excellent perforation sensitivity and an excellent printing quality, i.e., resolution and density of a printed image.
As heat-sensitive stencil paper, a laminate of a thermoplastic resin film such as polyester film and thin porous paper is conventionally known. The film used for this purpose is required to have a high thermal perforation sensitivity. In other words, the film is required to melt with a small quantity of heat and to have an adequate heat shrinkage ratio which enables openings of an appropriate size for producing a clear printed image to be formed.
As a material of a film used for such a purpose, a mixture of polyesters is known (e.g., Japanese Patent Application Laid-Open (KOKAI) No. 3-39294 (1991)). In the film material, polymers are mixed so as to lower the melting point and to enable perforation with a small quantity of heat. However, since the melting temperatures are different between the mixed polyesters, uniform perforation is not always possible.
A copolymerized polymer may be used as a material of this type of film. In most case, however, the crystallinity is lowered due to copolymerization, and when the film is industrially produced from such a copolymer, fusion is caused while the resin is heated and dried, so that melt extrusion by a screw extruder is impossible.
As a result of various studies undertaken by the present inventors so as to solve the above-described problems, it has been found that a biaxially oriented film which is formed from at least two polyesters and has a specified thickness and melting point is suitable for heat-sensitive stencil paper. On the basis of this finding, the present invention has been achieved.